The present invention is directed to a circuit that senses a temperature of a circuit board in a ballast of a high-intensity discharge (HID) lamp.
A HID lamp has an arc discharge contained within an arc tube and may be, for example, a mercury vapor, metal halide, low and high-pressure sodium, or xenon arc lamp. A HID lamp requires a ballast to start and maintain operation. A ballast is a device that provides the proper starting and operating electrical conditions. An electronic ballast uses solid state electronic circuitry to provide these conditions. Such solid state circuitry is well known and is not the subject of the present invention. However, one part of this circuitry is relevant to the present invention; in particular, the microcontroller.
A microcontroller is a computer-on-a-chip that is used to control an electronic device, such as the ballast of a HID lamp. The microcontroller is a single integrated circuit with a central processing unit, storage, peripherals as appropriate for the particular application, and connection pins for communicating signals and/or data. The circuitry for the ballast, including the microcontroller, is typically mounted on a printed circuit board (PCB).
During operation of a HID lamp, the PCB of the ballast may reach a temperature that can be harmful to the ballast or degrade ballast performance. The temperature of the PCB is monitored so that responsive steps can be taken as needed. The ballast microcontroller can be used to monitor the PCB temperature, provided the microcontroller has enough analog to digital (A/D) input pins available to be dedicated to temperature measurement functions.
If there are not enough microcontroller pins for the dedicated temperature measurement function, a strictly analog temperature compensation circuit can provide this function without involving the microcontroller. However, the degree of control is limited to one mode of operation and the analog compensation circuit is usually more sensitive to unit-to-unit variability than a microcontroller-based technique. Alternatively, a microcontroller with more pins can be provided, but the cost for this additional capability is higher and the additional pins increase the size of the microcontroller so as to require more real estate on the PCB.